PURPOSE: To prevent plate thickness variation and to reduce the pump driving power by detecting an oil film thickness in a static pressure bearing, comparing the film thickness with a standard thickness, and controlling a supply amount of high pressurized oil based on the deviation value found by the comparison.

CONSTITUTION: A non-contact displacement gage 24 is mounted on a bushing 4 of a static pressure bearing for a backup roll 1 and a servo motor 26 for supply amount adjustment of a high pressurized oil supply pump 12 and an oil film thickness gage setter 27 are also installed. A required oil film thickness is previously set by the setter 27 and an actual oil film thickness during rolling is detected by the gage 24. Then, a deviation of the detection value of the oil film thickness from the standard oil film thickness by the setter 27 is found and a high pressurized oil supply amount of the pump 12 is adjusted through the motor 26 according to the deviation amount. Thus, an oil film thickness in the bearing is kept constant under a rolling load changing condition. Therefore, plate thickness variation is prevented and the driving power of the pump 12 is reduced.